The US Food and Drug Administration has cleared use of Merck & Co’s immunotherapy Keytruda to treat cancer patients identified as having a biomarker called microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR).

Microsatellite instability - or MSI - is caused by a deficiency in the cell's ability to repair errors in the DNA sequence that occur during cell division leading to a characteristic change in microsatellite repeats. MSI-H is already an established biomarker in certain types of cancer.

Tumours with these biomarkers are most commonly found in colorectal, endometrial and gastrointestinal cancers, but also less commonly occur in those arising in the breast, prostate, bladder, thyroid gland and other places.

The FDA’s accelerated approval of Keytruda (pembrolizumab) in this setting marks the first time the agency has cleared a cancer treatment based on a common biomarker rather than the location in the body where the tumour originated.

It rides on the back of clinical data showing that, of 149 patients who received Keytruda in trials, 39.6 percent had a complete or partial response, and for 78 percent of those patients, the response lasted for six months or more.

The decision allows doctors to prescribe the drug to patients with solid tumours that have progressed following prior treatment and who have no satisfactory alternative treatment options, and patients with colorectal cancer that has progressed following treatment with certain chemotherapy drugs.

Meanwhile, the regulators also granted a priority review to Merck’s application to market Keytruda for the treatment of recurrent or advanced gastric or gastro-oesophageal junction adenocarcinoma.

The drug, a humanised monoclonal antibody that blocks the interaction between PD-1 and its ligands, thereby activating T lymphocytes, is already approved in the US for indications within melanoma, lung cancer, head and neck cancer and classical Hodgkin lymphoma (cHL).